1. Field of the Invention
The present invention relates to a semiconductor laser-device which is used in applications, such as a light source for wireless communication and sensing, where the device emits a light beam into a space and a human may see the light beam directly from its light source, and which ensures the safety of eyes. The present invention also relates to an optical transmission device and an optical transmission system using the above-described semiconductor laser device. The present invention still further relates to an electronic device, a control device, and a communication device using the above-described optical transmission device or optical transmission system, such as a personal computer, a personal digital assistant, or a digital camera. The present invention further relates to a space optical transmission method and a data transmission and reception method. Moreover, the present invention is directed to a semiconductor laser device emitting a light beam having an enlarged spot; an optical transmission device and an optical transmission system capable of simultaneous transmission and reception; an electronic device and a fiber optical transmission system, a control device, and a communication device using a fiber optical transmission system for multi-channel cable television (CATV) or audio-visual (AV) equipment; and a fiber optical transmission method and a data transmission and reception method.
2. Description of the Related Art
At present, space optical transmission using infrared light is widely used owing to standardization and popularization by the Infrared Data Association (IrDA). For example, FIG. 15 shows an example of the space optical transmission with which data is transferred between a personal computer 1500 and a personal digital assistant 1502. Using infrared light 1501, data such as addresses or schedules can be transferred over distances of up to about 1 m.
Recently, the transmission speed is ever growing with an increase in data capacity, changing from 1 Mbps to 4 Mbps, and further to 16 Mbps. There is also a growing demand for an increase in transmission distance. The transmission at distances of up to about 8 m is possible in the IrDA control standard. In this case, the transmission speed is limited to about 75 Kbps. Further, there is an effort to transmit image information on wireless communication.
Significant progress has been made in optical transmission technology using a fiber transmission path such as an optical fiber. In particular, plastic optical fibers are rapidly becoming widespread because of their low cost and large fiber diameters. The large fiber diameter makes it easy to couple with a light emitting element, as compared with a single mode fiber. In the optical transmission technology using the POF, 100 Mbps or more is presently established using a semiconductor laser.
At present, light emitting diodes (LED) are used as light sources in the space optical transmission. Now commercially available LEDs are not suitable for higher speed and longer distance transmission in view of the following two points:                (1) high-speed modulation is impossible due to the limitations of high-speed response characteristics of the LED: and        (2) power consumption is enormous in long distance transmission.        
As to the problem (1), the limit of a modulated frequency of typical LEDs is about 50 MHz. It is difficult to obtain a modulated frequency higher than about 50 MHz. Moreover, special LEDs capable of high-speed modulation dissipate very high power. As to the problem (2), for example, assuming that data in transmitted over a distance of 5 m at 40 Mbps, an LED alone dissipates as much as 1 W of power.
In contrast, semiconductor laser devices are capable of high-speed modulation. Further, the semiconductor laser devices require lower power consumption to obtain the same light beam output as compared with the LEDs. However, the semiconductor laser light beam itself exceeds a safety level of eyes, so that it is not allowable to emit the semiconductor laser light beam directly into a space. Moreover, the reliability of the semiconductor laser devices may be deteriorated during the high-output operation.
Furthermore, the space optical transmission system is not capable of full-duplex transmission. This in a serious problem. The problem will then be described below.
The electronic devices such as the personal computer 1500 and the personal digital assistant 1502 shown in FIG. 15 include a transmission and reception unit including a set of a transmission unit having an LED and a reception unit having a light receiving element. An electronic device communicates via a set of a transmission unit and a reception unit of another electronic device which may be the party on the either end of the communication. FIG. 16 shows an example of a transmission and reception unit 1600 into which a set of a transmission unit 1601 and a reception unit 1602 are integrated. This is an attempt to realize small-size and low-cost IrDA parts. Each unit is covered with a molded resin. Particularly, a resin material which does not transmit visible light is used for the reception unit 1602 in order not to be affected by noise due to background light. A typical LED has a wavelength band of 850-900 nm. Therefore, if visible light is blocked in the above-described manner, the background light noise can be reduced.
In the transmission and reception unit 1600, the transmission unit. 1601 emits a transmitted light beam 1603 having a directivity angle of about 30°. The transmitted light beam 1603 travels to a reception unit (not shown) of the party on the other end of communication (hereinafter referred to as the “other party”), its intensity being attenuated inversely with the second power of the distance. Part of the transmitted light beam 1603 also reaches the reception unit 1602 adjacent to the transmission unit 1601 and is received by a light receiving element in the reception unit 1602. The amount of light detected by the reception unit 1602 is small. Nevertheless, the intensity of such light is typically greater than or equal to that of a signal light beam from the other party, since the signal light beam is attenuated during transmission. Accordingly, when two-way communication is tested using two transmission and reception units, full-duplex communication cannot be attained. In this case, no more than half-duplex communication is performed, so that the effective transmission speed is greatly reduced.
Next, problems with the fiber optical transmission system will be described.
FIG. 17 shows an example of a transmission and reception unit 1700 at one end of the fiber optical transmission system using the POF. The transmission and reception unit 1700 includes a semiconductor laser chip 1715 in one package and a light receiving element 1705 in another package which are coupled with the POF 1716 and the POP 1717, respectively. The POP 1716 and the POP 1717 are fusion spliced into a single fiber 1718 reaching to the other party. In the optical transmission system, the transmission and reception unit at the other party employs a semiconductor laser chip and a light receiving element having the same characteristics as that of the transmission and reception unit 1700. The optical transmission system performs the half-duplex communication where one transmission and reception unit performs only transmission while the other transmission and reception unit performs only reception.
The semiconductor laser chip 1701 emits a light beam having a very small spot of several micrometers. The POF 1716 has a large diameter. Therefore, the alignment of the semiconductor laser chip 1701 and the POF 1716 is easier as compared with the single mode fiber. It is not as easy as the alignment of the LED and the POF. In the case of the full-duplex communication, two POFs are required and the two POFs must be fully separated. It is not possible to establish the full-duplex communication using a single fiber.